Differential pumping is the process of pumping an enclosed region to a known pressure while maintaining a second, connected region at a different pressure. The regions are typically connected by apertures and tubes with well-characterized conductances. Sufficiently high pumping speed is required to maintain this desired differential pressure. Differential pumping, for example, is required for high resolution electron microscopy. An electron source in a high resolution scanning electron microscope (SEM) must operate in ultra-high vacuum (UHV) while a sample under test (SUT) is in high vacuum. An electron beam from a UHV source chamber travels to a low vacuum sample chamber through an aperture with a known conductance. UHV at the source is maintained by pumping at a rate that is greater than the leak rate to the sample chamber through the aperture. The conductance is determined by diameter and length of the aperture and can be designed to hold a three to five magnitude pressure differential between the two vacuum regions when sufficiently high pumping speed is maintained.
However, conventional differential pumping techniques cannot be employed in a miniature scanning electron microscope due to extremely small size of the column and a very short distance between the electron source and the column elements.
Accordingly, a new apparatus and method are needed to maintain appropriate vacuums in miniature electron beam columns and other miniature charged particle devices.